Once a PDF file is attached, can I make changes to it?

No, PDF files (other than the fillable forms) are not editable. To make changes to an attached PDF File, you must:

(1)Remove the attached PDF file(s) from EFS-Web;
(2)Open the original editable file and make any necessary changes;
(3)Convert the file to PDF; and
(4)Re-attach the file in EFS-Web.

USPTO FAQ - Questions and Answers (23NOV2007)